The heart
The heart beats about 100,000 times a day, pumping roughly 7,000 litres of blood. Its structure and electrical system make this possible.
Location and coverings
The heart sits in the mediastinum, between the lungs, slightly left of centre, about the size of a fist. It is enclosed in the pericardium, a double-layered sac with fluid that reduces friction. The heart wall has three layers: the epicardium outside, the thick muscular myocardium, and the endocardium lining the chambers.
Chambers and valves
The right atrium receives oxygen-poor blood from the body through the venae cavae; the right ventricle pumps it to the lungs through the pulmonary trunk. The left atrium receives oxygen-rich blood from the pulmonary veins; the left ventricle pumps it to the body through the aorta. Atrioventricular valves (tricuspid on the right, mitral on the left) and semilunar valves (pulmonary and aortic) keep blood flowing one way.
The path of blood
Body -> venae cavae -> right atrium -> tricuspid valve -> right ventricle -> pulmonary valve -> pulmonary arteries -> lungs -> pulmonary veins -> left atrium -> mitral valve -> left ventricle -> aortic valve -> aorta -> body. The pulmonary circuit goes to the lungs; the systemic circuit goes everywhere else.
Coronary circulation
The heart muscle has its own blood supply through the coronary arteries, which branch from the aorta just above the aortic valve. Blockage of a coronary artery, usually by a fatty plaque and clot, starves part of the myocardium of oxygen: a heart attack (myocardial infarction). Dead heart muscle is replaced by scar tissue.
The conduction system
Cardiac muscle can generate its own impulses. The sinoatrial (SA) node in the right atrium is the natural pacemaker, setting a rate of about 60-100 beats per minute. The impulse spreads through the atria, is briefly delayed at the atrioventricular (AV) node so the atria can finish emptying, then races down the bundle of His and Purkinje fibres, making the ventricles contract from the bottom up.
The ECG
An electrocardiogram records the heart's electrical activity. The P wave shows atrial depolarisation; the QRS complex shows ventricular depolarisation (it hides atrial repolarisation); the T wave shows ventricular repolarisation. Changes in these waves help diagnose rhythm problems and heart attacks.
The cardiac cycle
Each heartbeat has a contraction phase (systole) and a relaxation phase (diastole). The 'lub' sound is the AV valves closing as the ventricles start to contract; the 'dub' is the semilunar valves closing as they relax. Heart murmurs are extra sounds, often from leaky or narrowed valves.
Cardiac output
Cardiac output is heart rate times stroke volume, the volume pumped per beat. At rest, about 70 beats per minute times 70 mL is roughly 5 litres per minute, the whole blood volume each minute. During exercise, sympathetic nerves and adrenaline raise both, and output can rise four- to sevenfold in trained athletes.
Key takeaways
The heart has a pericardium and three wall layers. The right side pumps to the lungs, the left to the body, with valves preventing backflow. Coronary arteries feed the myocardium; their blockage causes heart attacks. The SA node paces the heart; the AV node delays the impulse. The ECG shows P, QRS, and T waves. Cardiac output equals heart rate times stroke volume.
Flashcard
Sac around the heart
Check your understanding
A person's heart rate is 75 beats per minute and stroke volume is 80 mL. What is their cardiac output?